1. Field of the Disclosure
The disclosure relates generally to broadband wireless access communication systems, and more particularly to base station quantization in a broadband wireless access communication system.
2. Introduction
In WiMAX (IEEE 802.16), two principle communication wireless network nodes are defined including a Base Station (BS) and a Mobile Station (MS). WiMAX offers an open standard and support to quality of service (QoS) for different categories of services. The communication between the BS and MS is through an uplink channel and a downlink channel. In order to maintain this quality of service, multiple transmit and receive antenna arrays, commonly called MIMO systems, may employ beamforming techniques to compensate for channel conditions and for increasing signal-to-noise (SNR) levels at the MS. In one form of closed-loop MIMO systems, indices from a codebook of beamforming vectors or matrices known by both the BS and MS are determined at the MS and recommended to be used by the BS in the beamforming technique. This technique is known as codebook-based feedback (CBF) because the MS indicates the optimum precoding matrix (from the predefined codebook) to the BS via a feedback channel (e.g., the uplink channel). The codebook size increases significantly with the number of transmit antennas at the BS and may even increase with the number of transmitted data streams.
CBF is computationally expensive for MSs and the computational cost increases with the increase of the number of transmit antennas at the BS and the codebook size. Direct covariance feedback (DCOVF) and uplink channel sounding (ULCS) have been proposed as alternatives to CBF and offer lower complexity at the MS. Both DCOVF and ULCS, however, require dedicated pilots in the beamformed transmission from the BS. Dedicated pilots are pilots typically intended for only one user and are reference signals that are beamformed with the same transmit beamforming weights used on the data for that user. Thus, the dedicated pilots can only be used for the one user and are unavailable to other users for channel estimation purposes. Dedicated pilots, however, will limit the channel estimation performance at the MS because the MSs can only use the small subset of the total number of pilot symbols that are beamformed for that MS (this is opposed to broadcast pilots where all pilots can be used for channel estimation). In addition, dedicated pilots may be restricted in usage due to other constraints imposed by the control channel, or other pilot signals such as the midamble, etc.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a mechanism that lowers the computational complexity at the MS in closed-loop MIMO while not limiting the channel estimation performance at the MS.